Recent genome-wide screens with multiple viruses have identified several host proteins, which inhibit RNA virus accumulation and act as restriction factors against viruses. Unfortunately, the restriction functions of these host factors are not yet defined for important human viruses. Among the identified inhibitory host proteins are the family of WW-domain proteins, which are effective against several viruses. WW-domain proteins are abundant proteins participating in protein interactions and they are involved in several diseases, such as human cancers, inherited diseases and viral infections. Progress in our understanding of the mechanisms of host factors is slow due to functional redundancy or lack of knowledge about their functions. However, easily tractable virus - host systems, such as Tomato bushy stunt virus (TBSV) and yeast as a model host can contribute to our understanding of the functions of these host proteins. This project will likely advance our understanding of the role of host WW-domain proteins in virus-host interactions. This advance could immensely help other scientists working with less tractable, but devastating viral pathogens for which similar studies are currently not yet feasible. The following are the major strengths of the proposal: (i) Host restriction factors have clear negative impact on viruses. (ii) The restriction function of WW-domain proteins will be explored against tombusviruses and insect nodaviruses. (iii) The combination of yeast and authentic cell-free assay developed by the PI is currently the most potent for studying the mechanism of host factors involvement in viral RNA replication and viral pathogenesis. (iv) The research holds promise of benefiting society by leading to groundbreaking results in the area of virus replication, host-virus interactions and the viral pathogenesis.